Fluid pump devices of the general type under consideration are known and are used in various technology areas. One important application of such devices is in vehicle brake systems in which the fluid employed is hydraulic fluid for operating the systems.
In some vehicle brake systems, the hydraulic fluid is compressed to a pressure of up to 200 bar. To build up this pressure, the hydraulic fluid to be displaced is first delivered to a pressure space. As a result of piston movement, the incompressible hydraulic fluid is brought to the desired pressure. During subsequent expansion, air dissolved in the hydraulic fluid may be expelled, with the result that the pump device cannot build up any discharge capacity or can build up only a reduced discharge capacity. As a direct consequence, the brake system can no longer be operated or can no longer be operated with the desired degree of safety.
At low temperatures, the presence of air in the hydraulic fluid is critical. At low temperatures, the viscosity of the hydraulic fluid rises, and it therefore subjects the device to greater resistance. Consequently, the discharge capacity of the pump device or the required volume of hydraulic fluid falls. The probability increases that, in the lines of the brake system, areas are formed that consist only of air; and therefore the probability that the brake systems fails or is operated only with reduced power also increases. The problem is exacerbated in that, at low temperatures, the seals of the brake system can shrink and therefore more air can enter.
The quantity of air dissolved in the hydraulic fluid can be minimized by filling the brake system with the hydraulic fluid under a vacuum. However, this is a complicated process and affords advantages only when the brake system is also effectively sealed off so that no air can enter. Furthermore, this process has the added disadvantage that it is not particularly susceptible of being effected outside of a manufacturing facility when the hydraulic fluid needs to be exchanged.
Another way to minimize the absolute quantity of air present in the brake system is to reduce as much as possible the volume of detrimental space. Detrimental space is that portion of the pressure space that remains when the piston of the pump device is at its first end point (i.e., top dead center). With the reduction in detrimental space, the volume of the hydraulic fluid to be displaced also decreases, with the result that the air quantity expelled during expansion is reduced. However, reducing the volume of the detrimental space (or “detrimental volume”) presents hurdles, since a certain construction space is required for the accommodation and mounting of, for example, the closing element of the pump device. Thus, for example, while DE 10 2004 037 146 A1 describes a piston pump and attempts to reduce the detrimental volume, the described device still includes multiple components located in the detrimental space (e.g., a restoring spring and a disk-like holding element). The need to use these components places a specific limit on the minimization of the detrimental volume.